Disoxidants, i.e., oxygen absorbents, are widely employed for oxygen removal in various fields in preservation of foods and other products in which the presence of oxygen is not desired. For example, an iron powder as disclosed in JP-A-62-234544 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") has been of frequent use as an oxygen absorbent. However, a powdered oxygen absorbent, especially a fine powder, readily absorbs oxygen in air. In particular, iron-based oxygen absorbents are difficult to handle due to spontaneous combustibility.
JP-B-62-54704 (the term "JP-B" as used herein means an "examined published Japanese patent application") proposes a method of packaging a powdered oxygen absorbent with an air-permeable material. There remains a problem, however, that the powder leaks out unless it is completely sealed in. Further, since an oxygen absorbent conducts oxygen absorption from the surface of the air-permeable packaging material and undergoes reaction and hardening on the surface portion of the powder mass, it is not expected that the absorption reaction proceeds to the inside of the powder mass. As a result, oxygen absorbency cannot be manifested in proportion to the amount of the filled absorbent, thus failing to sufficiently control the rate of oxygen absorption.
JP-A-55-116436 also proposes a method of packaging an oxygen absorbent. However, this method not only encounters extreme difficulty in precisely and speedily packaging a small amount of an oxygen absorbent but also requires an expensive automatic filling and packaging machine. In addition, since such packaged oxygen absorbents are packed together with foods in many cases, cases may occur in which the oxygen absorbent is eaten by mistake.
Oxygen absorbents of sheet type include a sheet prepared from a blend of an activated iron oxide oxygen absorbent and a thermoplastic resin, e.g., polyethylene, as disclosed in JP-A-55-44344; a porous sheet or film base with its pores filled with an oxygen absorbent as disclosed in JP-A-55-109428; and a foamed polyurethane sheet having continuous cells in which an oxygen absorbent powder is buried as disclosed in JP-A-60-183373.
However, the oxygen absorbent of the sheet type comprising a thermoplastic resin and an oxygen absorbent does not always exhibit sufficient oxygen absorbency. Further, the oxygen absorbent made of porous sheet or film base has disadvantages in that the absorbent filled in pores of the porous base is easily released if so handled, and provision of an air-permeable film for prevention of such a release increases the cost. Furthermore, the oxygen absorbent made of foamed polyurethane sheet has disadvantages that it is difficult to stably obtain a continuous cellular structure on an industrial scale, and materials per se are too expensive for practical use.